Email Encryption

What is email encryption?

Email encryption is the process of encrypting, or disguising, the content of email messages to protect sensitive information from being read by unintended recipients, or cybercriminals looking to steal valuable information. The fact of the matter is that email continues to be the primary attack vector for cybercriminals. As such, it's important for organizations of all sizes to practice proper email security hygiene and utilize an email encryption solution when sending intellectual property or other sensitive information over email. Learn how to further secure your email environment by encrypting emails!

How does email encryption work?

SSL/TLS work by initiating a series of agreements between an email client — such as Gmail or Outlook — and a server to agree on the details of their connection. These agreements require a number of detailed steps, from determining what version of SSL/TLS will be used and how the communication will be encrypted to establishing that a secure connection is in place before transferring the data.

After the agreement series is completed, the email server returns a TLS digital certificate and public encryption key to the email client. The email client then verifies the certificate and creates a shared secret key (SSK), which is returned to the server. The server decrypts the SSK, which allows the transmission of emails.

With STARTTLS, however, it notifies a mail server that the contents of an email need to be encrypted. If the mail is intercepted, its contents and metadata are scrambled and difficult to decode. Once the transmission is received, the data will be decrypted.

Why is email encryption important?

Email often contains sensitive or confidential information. When email encryption is employed, it provides the essential security needed to protect that information. As such, organizations of all sizes often employ email encryption as a part of their cybersecurity strategies.

• Protect Sensitive Information – Encryption keeps confidential data—like personal, financial, or legal details—safe from unauthorized access during transmission.
• Prevent Data Breaches – By scrambling email content, encryption reduces the risk of interception or exposure during delivery across public or private networks.
• Support Regulatory Compliance – Many industries require secure communication under laws like GDPR, HIPAA, and PCI-DSS. Email encryption helps meet these mandates.
• Maintain Data Integrity – Encryption ensures messages aren’t altered in transit, preserving the accuracy and authenticity of email content.
• Build Trust with Recipients – Sending encrypted emails demonstrates a commitment to privacy and security, reinforcing trust with clients and partners.

Types of email encryption

Three standard email security protocols are used to secure email transmission:

Secure Sockets Layer (SSL)

SSL was one of the first protocols used to encrypt data between email clients and servers. It establishes a secure connection by initiating encryption before any data is transferred. While effective in its time, SSL is now deprecated due to known vulnerabilities and has been replaced by TLS in most modern systems.

Transport Layer Security (TLS)

TLS is the standard encryption protocol for securing email traffic today. It encrypts emails as they travel between mail servers, protecting against eavesdropping and tampering. TLS operates seamlessly in the background and is widely supported across modern email providers, making it essential for maintaining confidentiality and data integrity during transmission.

STARTTLS

STARTTLS is a protocol command that upgrades a plain-text connection (like SMTP, IMAP, or POP3) to a secure, encrypted one using TLS. It allows email systems to begin communication unencrypted and then switch to encryption if both servers support it.

While convenient, its security depends on proper configuration. Without enforced encryption, emails could still be sent unprotected if the receiving server doesn’t support TLS.

Because insecure email is a common attack vector for cybercriminals, it’s critical to use email security protocols like SSL/TLS and STARTTLS. Without this measure, users subject their emails and the sensitive data they contain to the possibility of interception, theft and email domain spoofing.

How to send encrypted email with Mimecast

Mimecast Secure Messaging protects sensitive data by making it easy for users to send and receive secure messages, and enabling policy-initiated secure messages at the email encryption gateway for an added layer of security.

Here’s how it works: